Generally, semiconductor devices are fabricated by depositing a plurality of insulating, conductive, and semiconductive material layers over a substrate or workpiece, and patterning the various material layers to form integrated circuits and electronic elements thereon. The conductive, semiconductive, and insulating material layers are patterned and etched to form integrated circuits (IC's).
To pattern a material layer, the material layer is deposited or formed over the workpiece or previously deposited material layers, and a layer of resist is deposited over the material layer. A pattern for the material layer is transferred to the layer of resist using lithography. For example, a photomask is typically used to image a master pattern onto the resist, by exposing the resist to light or energy through or reflected from a photomask. The resist is then developed, and the material layer is etched using the layer of resist as a mask. The resist is then removed, and additional material layers are deposited and patterned in a similar fashion. There may be a dozen or more lithography photomask levels required to manufacture an integrated circuit, for example.
As semiconductor devices decrease in size, as is the trend in the industry, patterning the various material layers becomes more difficult. As features become smaller, the wavelength used to develop the resist is decreased. For example, resists that develop at 193 nm are now being used, which provides a common Depth Of Focus (DOF) of less than 250 nm. The exposure latitude of a 193 nm resist is about 5%; thus, for most lithography processes, centering the dose and focus close to the optimum settings is desired, for critical dimension (CD) control and in order to reduce the number of reworks required and improve fabrication productivity.
Scanners are used in semiconductor device manufacturing to expose resist layers. A portion of a workpiece is typically exposed at a time, and the scanner steps from one portion of the workpiece to the next, repeating the process until the entire workpiece is exposed.
FIG. 1 illustrates a prior art lithography exposure tool or scanner 100. A workpiece 102 comprising a semiconductor wafer, for example, is placed on a support 104. Light, e.g., 193 nm laser light supplied by a light source 106, is directed through lenses 105 to a mirror 107 and is reflected to a lithography mask 108, through a lens system 110 towards the workpiece 102. The lens system 110 comprises a projection lens column having an array of a plurality of lenses 111 and 120 inside. There may be twenty or more lenses 111 and 120 disposed inside the lens system 110, for example. The workpiece 102 is moved in a first direction 112 and the mask 108 is moved in a second direction 114, the second direction 114 being opposite from the first direction 112. When the workpiece 102 is exposed to the projected light, an image of the mask 108 is formed on a resist layer on the workpiece 102.
If there is contamination on one or more lenses 111/120 of the lens system 110, light may scatter from the contaminated areas, referred to as stray light or scattered light. Stray light can be directed in any direction because it is not controlled by the lens system 110, for example.
Some semiconductor products are not very sensitive to stray light, such as 90 nm technologies or greater, e.g., having minimum feature size of about 120 nm and a pitch of about 240 nm. However, other semiconductor products are more sensitive to stray light, e.g., semiconductor products having an extremely small minimum feature size. In particular, in 65 nm technology and below, e.g., semiconductor devices having minimum feature sizes of 65 nm or less and a pitch of about 180 nm or less, are particularly sensitive to stray light.
If there is stray light present during the lithography process, the patterning of the workpiece 102 can be deleteriously affected. Stray light can destroy the pattern at a particular periodicity, for example. If there is a large amount of stray light in a lithography tool such as scanner 100 shown, the tool must be shut down so that the lens system 110 can be cleaned, requiring some production down time. The scanner 100 needs to be re-qualified after cleaning the lens 110, which may take a day, for example. Although contamination of the lens system 110 causes stray light, it is typically not practical to clean the projection lens or lens system 110 on a frequent basis to reduce stray light, because of the down time and loss of use of the production tool 100. Therefore, tool 100 vendors typically recommend that an upper specification limit of stray light be reached before the tool 100 is shut down to clean the lens system 110. The upper specification limit recommended is typically about 5%.
As stray light increases, a lower exposure dose is required to print a feature. Stray light illuminates regions on a wafer where illumination is not desired. Stray light affects CD control. It is common for stray light to be present in 193 nm and 248 nm lithography tools, for example, arising from lens contamination in the path of light, causing scattered light.
In a production line there are many chemicals used in a fabrication facility that can contaminate the lens system 110. Contamination sources include gases, evaporating chemicals, and organic materials emitting from the resist material. Other contamination sources include photo-induced deposition and particles in the equipment environment, as examples. Effects of stray light include a dose reduction for target CD, and unlike “hard” contaminations, such as large chunks of dust, stray light can result in a severe degradation of through pitch uniformity and a degradation of CD uniformity.
ArF scanners have been used as lithography exposure tools for a few years. ArF scanners utilize calcium fluoride optical elements and ArF laser light sources. ArF scanners allow better resolution and smaller targets without applying aggressive resolution enhancement or double exposure techniques. However, stray light can be a problem in ArF scanners, because of the calcium fluoride optics material, coatings, and the shorter wavelength used, which generate more stray light than prior art KrF scanners and other types of scanners, for example.
Thus, stray light is an issue that needs to be addressed in semiconductor lithography equipment.